Vote smart today and you can assist the strange birth of liberal Britain. Here's how. There is a liberal majority in Britain, but it is split between Labour and Liberal Democrats. This schism, in varying forms, has bedevilled the centre-left for generations. It helped the Tories to enjoy power at Westminster for 67 years out of a hundred in the last century.

Over the last three decades, politicians have made repeated bids to overcome it. There was the Lib-Lab pact of the late 1970s. There was the attempt by the newly founded Social Democrats to "break the mould" of British politics in the 1980s. In the 1990s, there was "the project" of overcoming the historic divide, pursued in far-reaching talks between the Liberal Democrats under Paddy Ashdown and New Labour under Tony Blair, and presided over by the historian-politician Roy Jenkins.

The project was scuppered by the scale of Labour's election victory on May 1 1997, and Tony Blair's subsequent failure to move towards a system of proportional representation - the only certain way of translating the liberal majority in the country into a liberal majority in parliament. According to Paddy Ashdown's riveting diaries, eight years ago tomorrow, on May 6 1997, Blair told him: "'My people are saying to me they can't see the point of having anything to do with you lot, since we have such a huge majority'. To which I reply: 'What the Lord giveth, the Lord taketh away'."

Well, let's leave the Lord out of it, shall we? But what the British electorate gave, it can take away. And should, today. Here is what can be done by every liberal-minded voter, every man or woman who believes in fairness, tolerance, decency and combining social justice with individual freedom. Before going to vote, check out the balance of forces in your constituency. If the leaflets stuck through the letterbox don't tell you, five minutes on the website of the Guardian or the BBC will. Where the Liberal Democrat is most likely to beat the Conservative, vote Lib Dem; where the Labour candidate is most likely to beat the Conservative, vote Labour; where neither has a chance, or the other is in second place, vote according to taste. It's as simple as that.

They call this tactical voting. Tactical has a pejorative ring: if you say of someone: "He's quite tactical", that is probably not a compliment. I call it intelligent voting. The overlap of shared liberal content in policy between Lib Dems and Labour is larger now than at any previous time. As Polly Toynbee noted in these pages yesterday: "There is more political difference within each party than between them". Go down the checklist of their policies, and you probably find yourself agreeing here more with one, and there more with the other. With the end of the great ideological conflicts of the 20th century, both parties are now seeking versions of capitalism with a human face.

If you had to describe the politics of Tony Blair in a single phrase, the best fit would still be "Gladstonian liberal". However, he has fallen down badly on three major issues: the Iraq war, civil liberties and Europe. The three perversions have at least one common root: the fact that after the terrorist attacks of September 11 2001 Blair bought far too readily and uncritically into the Manichean, crusading version of a Global War on Terror (GWOT, in Washington speak) peddled by the Bush White House. These are not small matters: peace, freedom and the future of our continent. On all three, the Lib Dems are better. Yet Labour leaders are right to warn that in some constituencies a substantial defection from Labour to the Lib Dems could bring in the Tory candidate. That would be the case in Putney, for example, to take just one marginal constituency I happen to know.

The result of intelligent voting, as suggested here, would probably not be a hung parliament, enabling the Lib Dems finally to secure a promise of proportional representation as the price of their support for Labour. Not this time. But it should lead to a diminished Labour majority and an increased Lib Dem presence in parliament.

This election is the closest thing we have seen to genuine three-party politics. Such a result would help to entrench three-party politics in the media and popular consciousness. It stands to reason that a Labour government would further lose popularity in its third term, even if it received a brief fillip from a Blair-to-Brown leadership transfer. If the Lib Dems performed strongly in parliament and the media, the trend could continue to the point where the logic which Blair saw, but failed to follow through, in the mid-1990s, would finally be acted upon, perhaps sometime between 2010 and 2014.

Seventy years ago, George Dangerfield published a book called The Strange Death of Liberal England, dating the death to the years 1910 to 1914. Many historians dispute his arguments, but no one has ever forgotten his title. This year Geoffrey Wheatcroft has published an entertaining book called The Strange Death of Tory England, suggesting that the Conservatives will suffer in the 21st century the fate suffered by the Liberals in the 20th. Having just picked up a Conservative election flyer which looks like an advertisement for Help The Aged, so white-haired are my local candidates for the county council, I might momentarily be inclined to agree. But I suspect he's wrong.

It's very important that the Conservatives, as led by Michael Howard and steered by the Australian dog-whistler Lynton Crosby, are roundly defeated this time. That's important not so much because of the content of their policies but because of the rancid, populist, Poujadist tone, appealing to the lower instincts of English xenophobia and nationalism. George Orwell once heard people in a pub breaking into a Cockney refrain that went: "For you can't do that there 'ere, No you can't do that there 'ere; anywhere else you can do that there, But you can't do that there 'ere." That, thought Orwell, was the British people's response to fascism. And that should be our response to Mr Crosby.

But a great part of the population of Britain, and especially of England, remains conservative with a small c and inclined to go with the large C, too. Humbled by a third election defeat, the Conservatives will eventually move back towards a more liberal-minded centre. That is already where many younger Conservatives are, in their private lives if not their public rhetoric. Even proportional representation would not - and obviously should not - secure a Lib-Lab majority for ever. One day, you would have to have a coalition government including the Conservatives. But in order to get there, Conservatives would need to become more liberal themselves, and would then be further constrained by the coalition. So the combined impact of Blair-Brownite New Labour and Lib Dem mould-breakers would have shifted the very centre of British politics, as Thatcherism did before them.

Here, then, is a scenario for the strange birth of liberal Britain. And I say Britain advisedly, not just England, because the growing national and ethnic diversity of 21st-century Britain is itself part of the story. But none of this is inevitable. To make it happen, we need to vote smart today.